Dual-type strain wave gearing

Abstract

An externally toothed gear of a dual-type strain wave gearing is provided with first and second external teeth having different teeth numbers, and a gap formed between these teeth as a cutter clearance area for tooth cutters. The maximum width L1 of the gap is 0.1 to 0.3 times the width L of the externally toothed gear. The depth from the tooth top land of the first external teeth to the deepest part of the gap is 0.9 to 1.3 times the depth of the first external teeth, and the depth from the tooth top land of the second external teeth to the deepest part of the gap is 0.9 to 1.3 times the depth of the second external teeth. The tooth bottom fatigue strength of the externally toothed gear provided with differing numbers of first and second external teeth is increased.

The invention claimed is: 
     
       1. A strain wave gearing comprising:
 a rigid first internally toothed gear formed with first internal teeth; 
 a rigid second internally toothed gear formed with second internal teeth, the second internally toothed gear being disposed so as to be coaxially aligned in parallel with the first internally toothed gear; 
 a flexible externally toothed gear in which first external teeth capable of meshing with the first internal teeth and second external teeth capable of meshing with the second internal teeth are formed in an outer-peripheral surface of a radially flexible cylindrical body, the second teeth differing in number from the first teeth, and the externally toothed gear being disposed coaxially inside the first and second internally toothed gears; and 
 a wave generator for flexing the externally toothed gear in an ellipsoidal shape to cause the first external teeth to partially mesh with the first internal teeth and to cause the second external teeth to partially mesh with the second internal teeth, 
 wherein a gap is formed between a tooth-trace-direction inner-end surface of the first external teeth and a tooth-trace-direction inner-end surface of the second external teeth, the gap having a prescribed width along a tooth trace direction, and the gap having a deepest part along a tooth depth direction at a tooth-trace-direction central portion; and 
 wherein the wave generator has a first wave bearing comprising a ball bearing for supporting the first external teeth, and a second wave bearing comprising a ball bearing for supporting the second external teeth; 
 0.1L<L1<0.3L, where L is a width from a tooth-trace-direction outer end of the first external teeth to a tooth-trace-direction outer end of the second external teeth, and L 1  is a maximum width of the gap along a tooth trace direction; and
   0.9 h 1< t 1<1.3 h 1 
   0.9 h 2< t 2<1.3 h 2 and 
     t (1)< t (2), 
 
 where h 1  is a tooth depth of the first external teeth, h 2  is a tooth depth of the second external teeth, t 1  is a distance from a tooth top land of the first external teeth to the deepest part of the gap, t 2  is a distance from a tooth top land of the second external teeth to the deepest part of the gap, t( 1 ) is a rim wall thickness of the first external tooth, and t( 2 ) is a rim wall thickness of the second external tooth, and 
 bearing-ball centers of the first wave bearing and the second wave bearing are located at positions that are equidistant, along the tooth trace direction, from a tooth-trace-direction center of the gap; 
 where an inter-ball-center distance Lo is a distance between the bearing-ball centers of the first and second wave bearings, and 
 the inter-ball-center distance is set so as to increase correspondingly with an increase in the maximum width L 1  of the gap, and to satisfy a relationship
   0.35 L<Lo< 0.7 L.    
 
 
     
     
       2. The strain wave gearing according to  claim 1 ,
 wherein a number of the first external teeth differs from a number of the first internal teeth, and 
 a number of second external teeth differs from a number of second internal teeth. 
 
     
     
       3. The strain wave gearing according to  claim 1 ,
 wherein a number of first external teeth is less than a number of first internal teeth, and 
 a number of first internal teeth and a number of second internal teeth are equal to each other. 
 
     
     
       4. The strain wave gearing according to  claim 1 
 wherein the wave generator causes the externally toothed gear to flex into an ellipsoidal shape so that the first external teeth are caused to mesh with the first internal teeth at two positions along a circumferential direction and the second external teeth are caused to mesh with the second internal teeth at two positions along the circumferential direction; and 
 a difference between a number of the first external teeth and a number of the second external teeth is 2n, where n is a positive integer.